This application claims the priority of German patent 197 226.1-26, filed Nov. 13, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process for operating a diesel engine using an automatic engine control which automatically controls the diesel engine as a function of characteristic diagrams between a rich operation and a lean operation.
From German Patent Document DE 43 34 763 A1, a process is known for operating an internal-combustion engine, in which an adsorption device for nitrogen oxides (NO.sub.x) is arranged in an emission control system. At an air-fuel ratio lambda (.lambda.) at which the exhaust gas flowing into the adsorption device is "lean", that is, when an overstoichiometric exhaust gas mixture with .lambda.&gt;1 is present, the adsorption device will adsorb NO.sub.x. The adsorbed NO.sub.x which was stored in this manner is released when the air-fuel ratio lambda of the exhaust gas flowing in the adsorption direction becomes "rich"; that is, when .lambda.&lt;1 or when the combustion exhaust gases are present at an understoichiometric ratio. In this known process, the temperature of the NO.sub.x -adsorption device is increased by injecting fuel into the exhaust gas and is lowered by blowing air into the exhaust gas in order to keep the working temperature of the NO.sub.x -storage device within a defined range.
From German Patent Document DE 195 43 219 C1, a process of the initially mentioned type is known in which a sensor arranged downstream of the storage catalyst is provided for sensing the NO.sub.x -concentration in the exhaust gas flow which, when an NO.sub.x storage threshold value is reached which, with respect to the characteristic diagram varies as a function of the rotational speed and the load, triggers a change-over from an operation of the diesel engine with .lambda.&gt;1 to an operation with .lambda.&lt;1. During an operation of the diesel engine with .lambda.&lt;1, the storage catalyst is regenerated and the nitrogen oxides NO.sub.x are reduced.
An object of the present invention is to develop a process of the initially mentioned type such that the NO.sub.x storage process and the regeneration process of a storage catalyst, through which the exhaust gases of the diesel engine flow, and the operating performance of the diesel engine are improved, particularly during the regeneration process.
This and other objects have been achieved according to the present invention by providing a process for operating a diesel engine having an automatic engine control which automatically controls the diesel engine as a function of characteristic diagrams between a rich operation and a lean operation, said process comprising: controlling a changeover between said rich operation and said lean operation as a function of predetermined change-over criteria via a computer; monitoring parameters necessary for said change-over criteria via a sensor system communicated with the computer; and selectively recalling separate characteristic diagrams and for the lean operation and the rich operation of the diesel engine stored in a memory which communicates with the computer.
This and other objects have been achieved according to the present invention by providing a system for operating a diesel engine, having an automatic engine control which automatically controls the diesel engine as a function of characteristic diagrams between a rich operation and a lean operation, said automatic engine control comprising: a computer which controls a changeover between said rich operation and said lean operation as a function of predetermined change-over criteria; a sensor system which communicates with the computer and monitors parameters necessary for said change-over criteria; and a memory which communicates with the computer and in which separate characteristic diagrams and for the operation of the diesel engine are stored for the lean operation and for the rich operation.
The invention is based on the general idea of providing separate characteristic diagrams or separate groups of characteristic diagrams for the lean operation as well as for the rich operation of the diesel engine, whereby an engine tuning is achieved which is optimally adapted to the respective operating performance. Although an understoichiometric combustion (.lambda.&lt;1) in the diesel engine necessary during the rich operation for forming reducing exhaust gas necessitates an engine tuning which is changed completely from that of the lean operation, particularly with respect to the fresh air feed and to the fuel injection operation, the present invention utilizes specifically worked-out characteristic curves to ensure an operating performance for a diesel engine which is expedient and advantageous also for the rich operation. For example, it is contemplated to design the engine tuning such that the soot emission values remain within acceptable limits, in particular, no visible smoke discharge taking place. Likewise, it is contemplated to tune the operating performance of the diesel engine by selecting special characteristic diagrams such that the driver and other vehicle occupants do not perceive the change-over between the different operating phases or the presence of different operating phases as being disturbing or do not even notice it at all. An aspect which is important in this respect is the smooth running of the diesel engine which can be kept within acceptable ranges by using specifically worked-out characteristic curves also in the rich operation.
In order to obtain a rich operation of the diesel engine, during which an adsorber system is regenerated which is connected behind the diesel engine, the diesel engine can be operated, for example, with a fuel feed which is increased in comparison to that of the lean operation.
Corresponding to a preferred embodiment of the process and system according to the invention, the characteristic diagrams for the rich operation of the diesel engine are selected for this purpose such that, in comparison to the lean operation, the fuel combustion in the respective combustion chamber takes place only relatively far after the top dead center position of the respective piston, that is, at relatively large crankshaft angles. The fuel combustion in the case of diesel fuels is composed of a precombustion (premixture combustion) and of a main combustion (diffusion combustion). Here, the late combustion essentially concerns the diffuse phase of the fuel combustion. Since the fuel combustion takes place late, it can no longer take place completely so that this is one of the reasons why the combustion air ratio lambda is reduced. However, this measure also deteriorates the efficiency of the engine such that clearly higher fuel quantities are required for being able to achieve the same engine power as in the lean operation. Since, in the case of a constant fresh air feed, the larger fuel quantity which exists in this phase can partially no longer be burnt, the combustion air ratio lambda is also reduced and the diesel engine is operated in a rich manner.
Corresponding to a further development of the process and system according to the invention, the point in time at which the diffusion combustion of the fuel takes place can be influenced by the point in time at which the main fuel injection starts and/or by the injection duration and/or by the injection quantity and/or by the injection pressure. For example, due to a late injection start, only a portion of the injected fuel quantity is burnt completely, whereas the remainder of the injection fuel quantity is essentially processed; that is, the long-chain fuel molecules are cracked to short-chain molecules; and is oxidized. In such a rich engine operation, this results in a considerably increased emission of unburnt hydrocarbons (HC) and carbon monoxide (CO). These intentionally achieved HC and CO reaction products are used as reducing agents on adsorber surfaces for converting nitrogen oxides NO.sub.x to nitrogen N.sub.2. Thus, during a rich engine operation, the exhaust gases have the reducing atmosphere required for regenerating the adsorber system.
If the injection parameters, such as the start, the duration, the quantity, and the pressure, are appropriately coordinated, the fuel fraction which does not participate in the development of the engine power, that is, the fuel fraction which forms the reducing atmosphere in the exhaust gas, can be varied in a limited range almost without any influence on the engine power. This has the result that, in this range of the rich operation, the diesel engine delivers approximately the same power as in the corresponding lean operation. The alternation between the operating modes can therefore take place without being noticed by the driver. At the same time, the above-mentioned range, in which the injected fuel quantity can be varied almost without any engine power change, permits a targeted influencing of the exhaust gas composition, particularly the adjustment of the reducing agent quantity to a desired value or to a value required in the respective operating point. Likewise, by suitable selection of the injection parameters, particularly the injection start and its duration, the ratio of hydrocarbons to carbon monoxide can be adjusted in a predetermined manner.
For improving the engine tuning, corresponding to an expedient further development of the process and system according to the invention, a preliminary (or pilot) fuel injection can take place before the main fuel injection, which preliminary fuel injection is changed in comparison to the lean operation with respect to the injection start and/or the injection duration. The preliminary fuel combustion also influences the operating performance of the diesel engine. Since, during the operation of the diesel engine according to the invention with a very late main fuel combustion, there may be vibrations and particularly a erratic running of the engine, in an expedient embodiment of the process and system according to the invention, the preliminary injection is coordinated with respect to the injection start and the injection operation with the delayed main or diffusion combustion such that essentially a uniform smooth running of the engine can again be ensured.
Through the above-mentioned measures for delaying the main fuel injection, the diffusion combustion is essentially influenced.
In another, particularly preferred embodiment of the process and system according to the invention, the point in time of the start of the main fuel injection is selected so late that a premixture combustion occurs in the case of a diesel engine with an exhaust gas recirculation in operating points with a relatively high exhaust gas recirculation rate. In this case, the injection point in time is preferably selected such that the diffusion combustion changes predominantly to a premixture combustion. This change from the diffusion combustion to the premixture combustion takes place because, when the diffusion combustion starts very late, relatively large quantities of unburnt hydrocarbons are formed, of which a large portion is again fed by way of the exhaust gas recirculation together with fresh air to the combustion spaces of the engine. Because of the compression taking place there, this results in a very good homogenized mixture which, when the ignition limit is reached, leads to a premixture combustion long before the upper dead center of the respective piston. In this case, combustion air ratios of .lambda.&lt;1 can also be implemented.
Corresponding to an expedient further development, in an injection performance in which the complete diffusion combustion changes to a predominant premixture combustion, additional reducing agent can be produced by a late post-injection of fuel. In this case, the fuel which was post-injected very late is essentially only still evaporated and, if at all, is cracked and/or burnt only to a small proportion. The evaporated fuel is used in the exhaust gas as an additional reducing agent which is produced inside the engine and which can be used for regenerating the adsorber system.
In another advantageous embodiment of the process according to the invention, after the main fuel injection, a fuel post-injection can take place which, particularly with respect to the start, the duration, and the quantity, is selected such that essentially only a processing, particularly a cracking of the afterinjected fuel, takes place in the combustion chamber. In this manner, additional reducing agent can be provided in the form of hydrocarbons HC in the exhaust gases for regenerating the catalyst. By this measure, it is, on the one hand, possible to vary the combustion air ratio lambda in a desirable manner. On the other hand, by post-injecting fuel in a targeted manner, the adsorber surface or catalyst surface can be heated because of an exothermal reaction taking place there. The power development of the diesel engine is not to be influenced by the post-injected fuel.
Another possibility of obtaining a rich operation of the diesel engine is achieved in another embodiment of the process and system according to the invention in that the characteristic diagram provides a premixture combustion at a point in time which, in comparison to the lean operation, is relative far before the top dead center of the respective piston. An early premixture combustion can be implemented particularly by a start of the preliminary fuel injection which is early in comparison to the lean operation.
In order to obtain an exhaust gas which has a reducing effect, instead of or in addition to the above-described measures through which a fuel quantity which is increased with respect to the lean operation is fed to the combustion, measures may also be applied in which an amount of fresh air is fed to the combustion which is reduced in comparison to the lean operation.
In an expedient embodiment of the process and system according to the invention, for this purpose, in the rich operation, the inlet cross-section in the intake system of each individual cylinder can be reduced because of correspondingly constructed characteristic diagrams. This can be implemented particularly in that, in the intake area of the respective combustion space, a throttle valve is provided which can be operated by way of an actuator by an auxiliary-force-operated actuating drive. This actuating drive is controlled according to the characteristic diagram as a function of signals of the automatic engine control.
An important parameter for the fresh air supply is the scavenging gradient. This is the pressure difference between the gas pressure which exists on the outlet side of the cylinders (exhaust back pressure) and the gas pressure which exists on the inlet side of the cylinders (intake pipe pressure). In a diesel engine with an exhaust gas recirculation device, the exhaust gas recirculation rate acts upon the fresh air supply such that an increased exhaust gas recirculation rate results in a reduced fresh air supply. In a particularly advantageous embodiment of the process and system according to the invention, the characteristic diagrams may provide for the rich operation of the diesel engine to increase the scavenging gradient. Since the exhaust gas recirculation pipe communicates with an exhaust gas collecting chamber behind the cylinder, the exhaust gas in the exhaust gas recirculation system essentially also has the exhaust gas back pressure. As a result, an increased scavenging gradient causes a rise of the exhaust gas recirculation rate and thus a reduction of the fresh air supply, which results in the desired effect of a reduced combustion air ratio lambda.
In order to increase the scavenging gradient, another embodiment of the process and system according to the invention suggests to throttle the fresh air supply of a diesel engine with an exhaust gas recirculation before the fresh air mixes with the exhaust gas recirculation flow. This reduces the pressure in the intake pipe and the scavenging gradient will rise. This has the result that, while the exhaust gas recirculation valve opening cross-section is the same, the exhaust gas quantity fed to the engine by way of the exhaust gas recirculation, that is, the exhaust gas recirculation rate, is increased.
In another embodiment of the process according to the invention, the scavenging gradient between the exhaust gas back pressure and the intake pipe pressure can also be increased in that the exhaust gas back pressure is increased, for example, with a throttle. In particular, it is possible in the case of a diesel engine with an exhaust gas turbocharger to reduce the turbine entrance cross-section and/or the exhaust gas volume flowing through the exhaust gas turbocharger. For this purpose, actuators may again be provided which can be operated by an auxiliary-force-operated actuating drive as a function of signals of the automatic engine control, particularly in a characteristic-diagram-controlled manner.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.